Removal of Chloride and Potassium from Kraft Chemical Recovery Cycles

Abstract

The current paper shows how a system for removal of both chloride and potassium may be incorporated in a potential future kraft chemical recovery system, where the traditional recovery boiler has been replaced by a black liquor gasifier. The suggested process involves cooling crystallization of sodium carbonate and sodium bicarbonate from a preferentially sulfide lean green liquor, leaving a stream depleted of useful chemicals but rich in chloride and potassium, that may be led to sewer. The investigation includes a modeling part, where a thermodynamic model has been used to calculate the solubility limits in the green liquor, and an experimental part, where both synthetic green liquor (with a controlled composition) and green liquor taken from a Swedish mill were used. The results show that the suggested process functions well and that both chloride and potassium will be selectively removed from the green liquor, while the useful chemicals will be recovered for reuse in the pulp mill. The size of the system necessary for the suggested process will depend on the concentrations of chloride and potassium that can be tolerated in the process liquor—an example is given showing the technical feasibility of the suggested process.